Semiconductor device and driving method thereof

ABSTRACT

A novel semiconductor device and a driving method thereof are provided. In the semiconductor device, a (volatile) node which holds data that is rewritten by arithmetic processing as appropriate and a node in which the data is stored are electrically connected through a source and a drain of a transistor whose channel is formed in an oxide semiconductor layer. The off-state current value of the transistor is extremely low. Therefore, electric charge scarcely leaks through the transistor from the latter node, and thus data can be held in the latter node even in a period during which supply of power source voltage is stopped. In the semiconductor device, a means of setting the potential of the latter node to a predetermined potential is provided. Specifically, a means of supplying a potential corresponding to “1” or “0” that is data stored in the latter node from the former node is provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device. In particular, the present invention relates to a semiconductor device to which power supply is stopped as appropriate during its operation. Further, the present invention relates to a driving method of the semiconductor device. Note that in this specification, a semiconductor device refers to a device which is capable of functioning by utilizing semiconductor characteristics.

2. Description of the Related Art

A semiconductor device such as a central processing unit (CPU) has a variety of configurations depending on its application. The semiconductor device generally has some kinds of memory devices such as a register and a cache memory as well as a main memory for storing data or a program. A register has a function of temporarily holding data for carrying out arithmetic processing, holding a program execution state, or the like. In addition, a cache memory is located between an arithmetic circuit and a low-speed main memory in order to reduce access to the main memory and speed up the arithmetic processing.

A register and a cache memory need to operate at higher speed in reading or writing data than a main memory. Thus, in many cases, a flip-flop is used as a register, and a static random access memory (SRAM) is used as a cache memory.

In order to reduce power consumption, supply of power source voltage to a semiconductor device is preferably stopped as appropriate (for example, in a period during which data is not input or output). Note that a flip-flop, an SRAM, and the like are volatile memory devices. Therefore, in the case where supply of power source voltage to a semiconductor device is stopped, data which has been lost in a volatile memory device such as a register or a cache memory need to be restored after the supply of power source voltage is restarted. For example, the following process is necessary: needed data is read from an auxiliary memory device provided in the outside of the semiconductor device to be newly written to the volatile memory device in a signal processing circuit. This leads to an operation delay of the signal processing circuit.

In view of this, a semiconductor device in which a nonvolatile memory device is located in the periphery of a volatile memory device has been developed. For example, Patent Document 1 discloses the following technique: data held in a flip-flop or the like is stored in a ferroelectric memory before supply of power source voltage is stopped, and the data stored in the ferroelectric memory is restored to the flip-flop or the like after the supply of power source voltage is restarted.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.     H10-078836

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide a novel semiconductor device and a driving method thereof. Specifically, an object is to provide a semiconductor device such as a central processing unit whose power consumption can be reduced and operation delay can be prevented.

In a semiconductor device of one embodiment of the present invention, a (volatile) node which holds data that is rewritten by arithmetic processing as appropriate and a node in which the data is stored are electrically connected to each other through a source and a drain of a transistor whose channel is formed in an oxide semiconductor layer. Note that the off-state current value of the transistor is extremely low. For that reason, electric charge scarcely leaks through the transistor from the latter node. As a result, data can be held in the latter node even in a period during which supply of power source voltage is stopped. Note that in this specification, stopping supply of power source voltage means not supplying the power source voltage (high power source potential or low power source potential) or making the power source voltage (a potential difference between the high power source potential and the low power source potential) zero or substantially zero.

The semiconductor device is provided with a means of setting the potential of the latter node to a predetermined potential. Specifically, the semiconductor device is provided with a means of supplying a potential corresponding to “1” or “0” that is data stored in the latter node from the former node.

For example, one embodiment of the present invention is a semiconductor device including: a first node capable of holding a potential corresponding to 1 or 0 as data in a period during which a power source voltage is supplied; a second node capable of holding the data regardless of whether the power source voltage is supplied; a first transistor which is configured to control whether to electrically connect the first node and the second node and in which a channel is formed in an oxide semiconductor layer; a second transistor a switching of which depends on a potential of the second node; and a means of supplying the potential corresponding to 1 or 0 to the second node regardless of the data.

Another embodiment of the present invention is a driving method of the semiconductor device, including: a first step of supplying the potential corresponding to 1 or 0 to the second node by the means; a second step of turning on the first transistor after the first step to store the data in the second node from the first node; a third step of stopping supply of the power source voltage after the second step; a fourth step of restarting the supply of the power source voltage after the third step; and a fifth step of restoring the data held in the first node before the second step depending on an on/off state of the second transistor after the fourth step.

Supply of power source voltage to a semiconductor device of one embodiment of the present invention is stopped as appropriate, so that the power consumption of the semiconductor device can be reduced. Further, data is stored before the supply of power source voltage is stopped and is restored after the supply of power source voltage is restarted, so that an operation delay can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a configuration example of a semiconductor device, and FIG. 1B is a flow chart illustrating an example of a driving method of the semiconductor device.

FIG. 2A illustrates a configuration example of a semiconductor device, and FIG. 2B is a timing diagram showing an example of potentials and signals supplied to the semiconductor device.

FIG. 3 illustrates a configuration example of a semiconductor device.

FIG. 4A illustrates a configuration example of a semiconductor device, and FIG. 4B is a timing diagram showing an example of potentials and signals supplied to the semiconductor device.

FIG. 5A illustrates a configuration example of an arithmetic portion, and FIG. 5B is a timing diagram showing an example of potentials and signals supplied to the arithmetic portion.

FIG. 6 illustrates a structure example of a transistor.

FIG. 7 illustrates a structure example of a semiconductor device.

FIGS. 8A to 8F each illustrate a specific example of an electronic device.

FIG. 9 illustrates a configuration example of a semiconductor device.

FIG. 10 illustrates a configuration example of a semiconductor device.

FIG. 11 shows measurement results of time required for data writing.

FIG. 12 shows measurement results of time required for data writing.

FIG. 13 shows a layout of a processor.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below in detail. Note that the present invention is not limited to the description below, and a variety of changes can be made without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited to the description below.

<Configuration Example of Semiconductor Device and Example of Driving Method Thereof>

FIG. 1A illustrates the configuration example of a semiconductor device of one embodiment of the present invention, and FIG. 1B is a flow chart illustrating an example of a driving method in data storing and data restoring of the semiconductor device illustrated in FIG. 1A.

The semiconductor device illustrated in FIG. 1A includes an arithmetic portion 1 that performs arithmetic processing and a data storing portion 2 where data held in the arithmetic portion 1 is stored.

The arithmetic portion 1 is supplied with a power source potential V1 and a power source potential V2. The arithmetic portion 1 can operate using a difference between the power source potentials V1 and V2 as a power source voltage. Note that here, the power source potential V1 is a high power source potential VDD or a low power source potential VSS, and the power source potential V2 is the low power source potential VSS. The arithmetic portion 1 has a node (Node_1) in which a potential corresponding to “1” or “0” can be held as data in a period during which the power source voltage is supplied.

The data storing portion 2 includes a transistor 21 a gate of which is electrically connected to a wiring supplying a signal OS_WE, one of a source and a drain of which is electrically connected to the node (Node_1) of the arithmetic portion 1, and the other of the source and the drain of which is electrically connected to a node (Node_2); a capacitor 22 one electrode of which is electrically connected to the node (Node_2) and the other electrode of which is electrically connected to a wiring supplying the power source potential V2; a transistor 23 a gate of which is electrically connected to the node (Node_2), one of a source and a drain of which is electrically to the wiring supplying the power source potential V2, and the other of the source and the drain of which is electrically connected to the arithmetic portion 1; and a means 24 capable of supplying a potential corresponding to “1” or “0” that is data held in the node (Node_1) in the arithmetic portion 1 to the node (Node_2). Note that the capacitor 22 is not necessarily provided in the data storing portion 2 illustrated in FIG. 1A.

Here, the potential of the signal OS_WE is higher than the sum of the threshold voltage of the transistor 21 and the high power source potential VDD, and the potential of the signal OS_WE is the low power source potential VSS (this case is referred to as a former case). Alternatively, the potential of the signal OS_WE can be the high power source potential VDD, and the potential of the signal OS_WE can be the low power source potential VSS (this case is referred to as a latter case). In the latter case, the power source potential needed for driving the semiconductor device can be reduced as compared with the former case. On the other hand, in the former case, the high power source potential VDD can be supplied to the node (Node_2), so that data storing described later can be performed surely.

Note that the transistor 21 is a transistor whose channel is formed in an oxide semiconductor layer. A path for supplying electric charge to the node (Node_2) is only a path through the source and the drain of the transistor whose channel is formed in the oxide semiconductor layer. The transistor whose channel is formed in the oxide semiconductor layer has an extremely low off-state current value. Therefore, in a period during which the transistor is off, the potential of the node (Node_2) can be kept substantially constant. For this reason, data can be held in the node (Node_2) regardless of whether the power source voltage is supplied. In other words, data held in the node (Node_1) in the arithmetic portion 1 can be stored in the node (Node_2).

The transistor 23 can be formed using any of a variety of semiconductor materials. For example, a material such as silicon or germanium can be used. Alternatively, it is possible to use a compound semiconductor or an oxide semiconductor. Note that as the transistor 23, a transistor whose mobility is high (for example, a transistor in which a channel is formed in single crystal silicon) is preferably used.

Data which has been held in the node (Node_1) in the arithmetic portion 1 and then lost by stop of supply of power source voltage can be restored in the arithmetic portion 1 depending on the on/off state of the transistor 23.

Hereinafter, an example of a driving method of the semiconductor device illustrated in FIG. 1A will be described with reference to FIG. 1B.

In the semiconductor device illustrated in FIG. 1A, the potential of the node (Node_2) can be initialized (S1). Specifically, a potential corresponding to “1” or “0” that is data held in the node (Node_1) in the arithmetic portion 1 can be supplied to the node (Node_2) by the means 24.

Next, data held in the node (Node_1) in the arithmetic portion 1 can be stored in the node (Node_2) in the data storing portion 2 (S2). Specifically, the transistor 21 is turned on, whereby the data held in the node (Node_1) is written to the node (Node_2). Since the potential corresponding to “1” or “0” is supplied to the node (Node_2), the potential of the node (Node_2) is not changed by the storing in some cases.

Next, supply of power source voltage to the semiconductor device can be stopped (S3). Specifically, the power source potential V1 is set to the low power source potential VSS.

Next, the supply of power source voltage to the semiconductor device can be restarted (S4). Specifically, the power source potential V1 is set to the high power source potential VDD.

Then, the data that has been held in the node (Node_1) in the arithmetic portion 1 can be restored (S5). Specifically, the data in the node (Node_1) is restored depending on the on/off state of the transistor 23.

As described above, supply of power source voltage to the semiconductor device illustrated in FIG. 1A is stopped, so that the power consumption of the semiconductor device illustrated in FIG. 1A can be reduced. Further, the data that has been held in the node (Node_1) is restored after the supply of power source voltage is restarted, so that an operation delay can be prevented.

In the semiconductor device illustrated in FIG. 1A, the potential of the node (Node_2) is initialized before the data is stored. In this case, the time required for storing the data can be short as compared with the case where the initialization is not performed. This point is described below in detail.

Storing data in the semiconductor device illustrated in FIG. 1A means that the potential of the node (Node_2) is changed into a potential corresponding to data held in the node (Node_1). That is, the potential of the node (Node_2) is rewritten depending on the data storing in the following manner: the potential corresponding to “1” is rewritten into the potential corresponding to “0”, or the potential corresponding to “0” is rewritten into the potential corresponding to “1”.

In the semiconductor device illustrated in FIG. 1A, the time required for rewriting the potential corresponding to “0” into the potential corresponding to “1” and the time required for rewriting the potential corresponding to “1” into the potential corresponding to “0” are not necessarily the same. Specifically, in the semiconductor device illustrated in FIG. 1A, data held in the node (Node_2) is identified on the basis of the on/off state of the transistor 23. That is, the data held in the node (Node_2) is identified on the basis of not only the potential of the node (Node_2) but also the threshold voltage of the transistor 23. For such a reason, both of the times depend on the threshold voltage of the transistor 23. For example, in the case where the threshold voltage of the transistor 23 is a value nearer to the potential corresponding to “0” than to the potential corresponding to “1”, the time required for rewriting the potential corresponding to “1” into the potential corresponding to “0” is longer than the time required for rewriting the potential corresponding to “0” into the potential corresponding to “1”. Further, in the case where there is a possibility that both the potential corresponding to “0” and the potential corresponding to “1” need to be rewritten in storing data, the longer time (the time required for rewriting the potential corresponding to “1” into the potential corresponding to “0”) is set as the time required for storing data. This is for surely storing data.

In the semiconductor device illustrated in FIG. 1A, the potential of the node (Node_2) is set in advance so as to correspond to “1” or “0”. That is, data which takes a longer time to be stored can be supplied to the node (Node_2) in advance. Accordingly, in the semiconductor device illustrated in FIG. 1A, the shorter time required for storing data can be set as the time required for storing data. That is, the time required for storing data can be shortened. As a result, in the semiconductor device illustrated in FIG. 1A, an operation delay can be prevented.

<Specific Example 1 of Means 24>

FIG. 2A illustrates the configuration example of a semiconductor device in which the means 24 is specifically shown. FIG. 2B is an example of a timing diagram of the power source potentials V1 and V2 and signals OS_WE and OS_WE2 supplied to the semiconductor device illustrated in FIG. 2A.

The semiconductor device illustrated in FIG. 2A includes, as the means 24, an analog switch 25 controlling whether to electrically connect the node (Node_1) in the arithmetic portion 1 and one of a source and a drain of the transistor 21 and an analog switch 26 controlling whether to electrically connect a wiring supplying the power source potential V2 and the one of the source and the drain of the transistor 21. Note that the analog switches 25 and 26 operate in an inverse manner with the signal OS_WE2 and an inverted signal of the signal OS_WE2 (an output signal of an inverter 27). That is, in the semiconductor device illustrated in FIG. 2A, a switch is formed of the analog switches 25 and 26 and the inverter 27. Here, the potential of the signal OS_WE2 is the high power source potential VDD or the low power source potential VSS.

In the semiconductor device illustrated in FIG. 2A, the potential of the node (Node_2) is initialized in periods (Periods T1 and T3 shown in FIG. 2B) prior to periods during which data is stored (Periods T2 and T4 shown in FIG. 2B). Specifically, the node (Node_2) is supplied with the power source potential V2 (the low power source potential VSS) through the analog switch 26 and the source and the drain of the transistor 21. Note that in the semiconductor device illustrated in FIG. 2A, Periods Ta and Tc shown in FIG. 2B are periods during which arithmetic processing can be performed in the arithmetic portion 1, and Periods Tb and Td are periods during which supply of power source voltage is stopped.

Here, the length of the periods during which the potential of the node (Node_2) is initialized (Periods T1 and T3 shown in FIG. 2B) is preferably longer than the length of the periods during which data is stored (Periods T2 and T4 shown in FIG. 2B). The reason for this is as follows: one of the potential corresponding to “0” into which the potential corresponding to “1” is rewritten and the potential corresponding to “1” into which the potential corresponding to “0”, which takes a longer time to be stored, can be supplied to the node (Node_2) in advance (in Periods T1 and T3).

Note that the analog switch 26 included in the semiconductor device illustrated in FIG. 2A may be replaced with a transistor 28 as illustrated in FIG. 3. Note that the switching of the transistor 28 is controlled with the signal OS_WE2.

<Specific Example 2 of Means 24>

FIG. 4A illustrates the configuration example of a semiconductor device in which the means 24 different from that in the semiconductor device illustrated in FIG. 2A is specifically shown. FIG. 4B is an example of a timing diagram of the power source potentials V1 and V2, the signal OS_WE, and a signal OS_WE3 supplied to the semiconductor device illustrated in FIG. 4A.

The semiconductor device illustrated in FIG. 4A includes, as the means 24, a transistor 29 controlling whether to electrically connect a wiring supplying the power source potential V2 and the node (Node_2). Note that the switching of the transistor 29 is controlled with the signal OS_WE3. The transistor 29 is a transistor whose channel is formed in an oxide semiconductor layer. Here, the potential of the signal OS_WE3 is the high power source potential VDD or the low power source potential VSS.

In the semiconductor device illustrated in FIG. 4A, the potential of the node (Node_2) is initialized in periods (Periods t1 and t3 shown in FIG. 4B) prior to periods during which data is stored (Periods t2 and t4 shown in FIG. 4B). Specifically, when the transistor 29 is turned on, the node (Node_2) is supplied with the power source potential V2 (the low power source potential VSS). Note that in the semiconductor device illustrated in FIG. 4A, Periods ta and tc shown in FIG. 4B are periods during which arithmetic processing can be performed in the arithmetic portion 1, and Periods tb and td are periods during which supply of power source voltage is stopped.

Here, the length of the periods during which the potential of the node (Node_2) is initialized (Periods t1 and t3 shown in FIG. 4B) is preferably longer than the length of the periods during which data is stored (Periods t2 and t4 shown in FIG. 4B).

<Example of Data Restoring in Arithmetic Portion 1>

FIG. 5A illustrates the configuration example of the arithmetic portion 1 included in the semiconductor devices illustrated in FIGS. 1A and 1B, FIGS. 2A and 2B, FIG. 3, and FIGS. 4A and 4B. FIG. 5B is an example of a timing diagram of the power source potentials V1 and V2, a signal CK, a signal Reset, and a signal OS_RD supplied to the semiconductor device illustrated in FIG. 5A. Here, the potential of each of the signal CK, the signal Reset, and the signal OS_RD is the high power source potential VDD or the low power source potential VSS.

The arithmetic portion 1 illustrated in FIG. 5A includes an inverter 10 outputting a signal to the node (Node_1); a NAND gate 11 to which the signal Reset is input as one input and the potential of the node (Node_1) is input as the other input; an analog switch 12 controlling whether to electrically connect an input terminal (In) of the arithmetic portion 1 and the node (Node_1); an analog switch 13 controlling whether to electrically connect an output terminal of the NAND gate 11 and each of an output terminal (Out) of the arithmetic portion 1 and an input terminal of the inverter 10; and an analog switch 14 controlling whether to electrically connect a source or a drain of the transistor 23 included in the data storing portion 2 and each of the output terminal (Out) of the arithmetic portion 1 and the input terminal of the inverter 10.

Note that the analog switch 12 operates in response to the signal CK and an inverted signal of the signal CK (output signal of an inverter 15). The analog switches 13 and 14 operate in an inverse manner with the signal OS_RD and an inverted signal of the signal OS_RD (an output signal of an inverter 16). That is, in the semiconductor device illustrated in FIG. 5A, a switch is formed of the analog switches 13 and 14 and the inverter 16.

In the arithmetic portion 1 illustrated in FIG. 5A, after supply of power source voltage is restarted, the potential of a signal output from the NAND gate 11 is the high power source potential VDD in a period during which the potential of the signal CK is the high power source potential VDD and the potential of the signal Reset is the low power source potential VSS (in Period T_Reset shown in FIG. 5B). Since the potential of the signal OS_RD is the low power source potential VSS in Period T_Reset, the potential of a signal output from the output terminal (Out) of the arithmetic portion 1 is the high power source potential VDD and the potential of the node (Node_1) is the low power source potential VSS. In a period during which the potential of the signal OS_RD is the high power source potential VDD after the potential of the signal Reset is changed into the high power source potential VDD (in Period T_RD shown in FIG. 5B), data that has been held in the node (Node_1) can be restored. Specifically, data that has been held in the node (Node_1) can be restored in such a manner that the potential of the node (Node_1) is raised to the high power source potential VDD when the transistor 23 is on, and the potential of the node (Node_1) is kept at the low power source potential VSS when the transistor 23 is off.

<Example of Transistor Whose Channel is Formed in Oxide Semiconductor Layer>

FIG. 6 illustrates an example of a structure of a transistor whose channel is formed in an oxide semiconductor layer. The transistor illustrated in FIG. 6 includes an oxide semiconductor layer 31 provided over a layer 30 having an insulating surface, a conductive layer 32 in contact with one end of the oxide semiconductor layer 31, a conductive layer 33 in contact with the other end of the oxide semiconductor layer 31, an insulating layer 34 provided over the oxide semiconductor layer 31 and the conductive layers 32 and 33, and a conductive layer 35 provided over the insulating layer 34. Note that in the transistor illustrated in FIG. 6, the conductive layers 32 and 33 function as a source and a drain, the insulating layer 34 functions as a gate insulating film, and the conductive layer 35 functions as a gate.

<1. Specific Example of Oxide Semiconductor Layer 31>

<(1) Oxide Semiconductor Material>

A film containing at least indium can be used as the oxide semiconductor layer 31. In particular, a film containing indium and zinc is preferably used. As a stabilizer for reducing variations in electric characteristics of the transistor, a film containing gallium (Ga) in addition to indium and zinc is preferably used.

Alternatively, a film which contains, as a stabilizer, one or more of tin, hafnium, aluminum, zirconium, and lanthanoid such as lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, or lutetium can be used as the oxide semiconductor layer 31.

As the oxide semiconductor layer 31, for example, a film of any of the following oxides can be used: indium oxide; a two-component metal oxide such as an In—Zn-based oxide, an In—Mg-based oxide, or an In—Ga-based oxide; a three-component metal oxide such as an In—Ga—Zn-based oxide, an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide; and a four-component metal oxide such as an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide.

Note that here, for example, an “In—Ga—Zn-based oxide” means an oxide containing In, Ga, and Zn as its main component and there is no particular limitation on the ratio of In:Ga:Zn. The In—Ga—Zn-based oxide may contain a metal element other than In, Ga, and Zn.

Note that part of oxygen included in the oxide semiconductor layer 31 may be substituted with nitrogen.

<(2) Crystal Structure of Oxide Semiconductor>

For the oxide semiconductor layer 31, a film having a single crystal state, a polycrystalline (also referred to as polycrystal) state, an amorphous state, or the like can be used. In addition, a CAAC-OS (c-axis aligned crystalline oxide semiconductor) film can be used for the oxide semiconductor layer 31. The CAAC-OS film is described in detail below.

The CAAC-OS film is one of oxide semiconductor films including a plurality of c-axis aligned crystal parts.

In a transmission electron microscope (TEM) image of the CAAC-OS film, a boundary between crystal parts, that is, a grain boundary is not clearly observed. Thus, in the CAAC-OS film, a reduction in electron mobility due to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a direction substantially parallel to a sample surface (cross-sectional TEM image), metal atoms are arranged in a layered manner in the crystal parts. Each metal atom layer has a morphology reflected by a surface over which the CAAC-OS film is formed (hereinafter, a surface over which the CAAC-OS film is formed is referred to as a formation surface) or a top surface of the CAAC-OS film, and is arranged in parallel to the formation surface or the top surface of the CAAC-OS film.

On the other hand, according to the TEM image of the CAAC-OS film observed in a direction substantially perpendicular to the sample surface (plan TEM image), metal atoms are arranged in a triangular or hexagonal configuration in the crystal parts. However, there is no regularity of arrangement of metal atoms between different crystal parts.

From the results of the cross-sectional TEM image and the plan TEM image, alignment is found in the crystal parts in the CAAC-OS film.

Most of the crystal parts included in the CAAC-OS film each fit inside a cube whose one side is less than 100 nm. Thus, there is a case where a crystal part included in the CAAC-OS film fits inside a cube whose one side is less than 10 nm, less than 5 nm, or less than 3 nm. Note that when a plurality of crystal parts included in the CAAC-OS film are connected to each other, one large crystal region is formed in some cases. For example, a crystal region with an area of 2500 nm² or more, 5 μm² or more, or 1000 μm² or more is observed in some cases in the plan TEM image.

A CAAC-OS film is subjected to structural analysis with an X-ray diffraction (XRD) apparatus. For example, when the CAAC-OS film including an InGaZnO₄ crystal is analyzed by an out-of-plane method, a peak appears frequently when the diffraction angle (2θ) is around 31°. This peak is derived from the (009) plane of the InGaZnO₄ crystal, which indicates that crystals in the CAAC-OS film have c-axis alignment, and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-plane method in which an X-ray enters a sample in a direction substantially perpendicular to the c-axis, a peak appears frequently when 2θ is around 56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal. Here, analysis (φ scan) is performed under conditions where the sample is rotated around a normal vector of a sample surface as an axis (φ axis) with 2θ fixed at around 56°. In the case where the sample is a single-crystal oxide semiconductor film of InGaZnO₄, six peaks appear. The six peaks are derived from crystal planes equivalent to the (110) plane. On the other hand, in the case of a CAAC-OS film, a peak is not clearly observed even when φ scan is performed with 2θ fixed at around 56°.

According to the above results, in the CAAC-OS film having c-axis alignment, while the directions of a-axes and b-axes are different between crystal parts, the c-axes are aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface. Thus, each metal atom layer arranged in a layered manner observed in the cross-sectional TEM image corresponds to a plane parallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of the CAAC-OS film or is formed through crystallization treatment such as heat treatment. As described above, the c-axis of the crystal is aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface. Thus, for example, in the case where a shape of the CAAC-OS film is changed by etching or the like, the c-axis might not be necessarily parallel to a normal vector of a formation surface or a normal vector of a top surface of the CAAC-OS film.

In the CAAC-OS film, distribution of c-axis aligned crystal parts is not necessarily uniform. For example, in the case where crystal growth leading to the crystal parts of the CAAC-OS film occurs from the vicinity of the top surface of the film, the proportion of the c-axis aligned crystal parts in the vicinity of the top surface is higher than that in the vicinity of the formation surface in some cases. Further, when an impurity is added to the CAAC-OS film, a region to which the impurity is added is altered, and the proportion of the c-axis aligned crystal parts in the CAAC-OS film varies depending on regions, in some cases.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed by an out-of-plane method, a peak of 2θ may also be observed at around 36°, in addition to the peak of 2θ at around 31°. The peak of 2θ at around 36° indicates that a crystal having no c-axis alignment is included in part of the CAAC-OS film. It is preferable that in the CAAC-OS film, a peak of 2θ appear at around 31° and a peak of 2θ do not appear at around 36°.

The CAAC-OS film is an oxide semiconductor film having a low impurity concentration. The impurity means an element other than main components of the oxide semiconductor film, such as hydrogen, carbon, silicon, or a transition metal element. In particular, an element (e.g., silicon) having higher strength of bonding to oxygen than a metal element included in the oxide semiconductor film takes oxygen away in the oxide semiconductor film to disrupt the atomic arrangement in the oxide semiconductor film, which causes a lowering of the crystallinity of the oxide semiconductor film. A heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (or molecular radius), and thus disrupts the atomic arrangement in the oxide semiconductor film when included in the oxide semiconductor film, which causes a lowering of the crystallinity of the oxide semiconductor film. Note that the impurity included in the oxide semiconductor film serves as a carrier trap or a carrier generation source in some cases.

The CAAC-OS film is an oxide semiconductor film having a low density of defect states. For example, oxygen vacancies in the oxide semiconductor film serve as carrier traps or serve as carrier generation sources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defect states is low (few oxygen vacancies) is referred to as “highly purified intrinsic” or “substantially highly purified intrinsic”. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier generation sources, and thus has a low carrier density. Thus, a transistor including the oxide semiconductor film rarely has a negative threshold voltage (rarely has normally-on characteristics). A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier traps. Thus, the transistor including the oxide semiconductor film has a small variation in electric characteristics and accordingly has high reliability. Charges trapped by the carrier traps in the oxide semiconductor film take a long time to be released and may behave like fixed charges. Thus, the transistor including the oxide semiconductor film with a high impurity concentration and a high density of defect states has unstable electric characteristics in some cases.

In a transistor using the CAAC-OS film, change in electric characteristics due to irradiation with visible light or ultraviolet light is small.

Note that the oxide semiconductor layer 31 may be a stacked film including two or more films of an amorphous oxide semiconductor film, a microcrystalline oxide semiconductor film, and a CAAC-OS film, for example.

<(3) Layer Structure of Oxide Semiconductor>

For the oxide semiconductor layer 31, not only a single-layer oxide semiconductor film but also a layer formed of a stack having plural kinds of oxide semiconductor films may be used. For example, a layer including at least two of an amorphous oxide semiconductor film, a polycrystalline oxide semiconductor film, and a CAAC-OS film can be used for the oxide semiconductor layer 31.

Alternatively, a layer formed of a stack of oxide semiconductor films having different compositions can be used for the oxide semiconductor layer 31. Specifically, a layer including a first oxide semiconductor film (hereinafter also referred to as an upper layer) which is provided on the insulating layer 34 side and a second oxide semiconductor film (hereinafter also referred to as a lower layer) which is provided on the layer 30 side and has a different composition from the first oxide semiconductor film can be used as the oxide semiconductor layer 31.

<2. Specific Example of Conductive Layers 32 and 33>

For the conductive layer 32 and the conductive layer 33, a film containing an element selected from aluminum, copper, titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium, a film of an alloy containing any of these elements, a film of a nitride containing any of these elements, or the like can be used. Alternatively, each of the conductive layer 32 and the conductive layer 33 can be formed with a stack of these films.

<3. Specific Example of Insulating Layer 34>

For the insulating layer 34, an inorganic insulating material film such as a silicon oxide film, a silicon nitride film, a silicon oxynitride film, a silicon nitride oxide film, an aluminum oxide film, an aluminum oxynitride film, or a gallium oxide film can be used. Alternatively, a stack of layers of these materials can also be used. In particular, an aluminum oxide film is preferably used for the insulating layer 34. The aluminum oxide film has a high shielding (blocking) effect of preventing penetration of both oxygen and an impurity such as hydrogen. Therefore, when the layer including an aluminum oxide film is used as the insulating layer 34, it is possible to prevent release of oxygen from the oxide semiconductor layer 31 and entry of an impurity such as hydrogen to the oxide semiconductor layer 31.

The insulating layer 34 can be formed using a film including a hafnium oxide film, an yttrium oxide film, a hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)) film, a hafnium silicate film to which nitrogen is added, a hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)) film, a lanthanum oxide film (i.e., a film formed of what is called a high-k material), or the like. By using such a film, gate leakage current can be reduced.

<4. Specific Example of Conductive Layer 35>

For the conductive layer 35, a film containing an element selected from aluminum, copper, titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium or a film of an alloy containing any of these elements as its component can be used. Alternatively, a metal oxide containing nitrogen, specifically, an In—Ga—Zn—O film containing nitrogen, an In—Sn—O film containing nitrogen, an In—Ga—O film containing nitrogen, an In—Zn—O film containing nitrogen, a Sn—O film containing nitrogen, an In—O film containing nitrogen, or a metal nitride (e.g., InN or SnN) film can be used for the conductive layer 35. Such a nitride film has a work function of 5 eV (electron volts) or higher, preferably 5.5 eV (electron volts) or higher, which enables the threshold voltage of the transistor to be positive when such a nitride film is used as the gate, so that what is called a normally-off switching element can be achieved. Alternatively, the conductive layer 35 can be formed with a stack of these films.

<5. Supplementary Note>

In the transistor illustrated in FIG. 6, it is preferred to suppress entry of impurities into the oxide semiconductor layer 31 and release of constituent element of the oxide semiconductor layer 31. This is because the electric characteristics of the transistor are changed when such phenomena occur. As a means of suppressing these phenomena, insulating layers having a high blocking effect are provided above and below the transistor (between the layer 30 having an insulating surface and the transistor, and over the insulating layer 34 and the conductive layer 35). For the insulating layers, for example, an inorganic insulating material film such as a silicon oxide film, a silicon nitride film, a silicon oxynitride film, a silicon nitride oxide film, an aluminum oxide film, an aluminum oxynitride film, or a gallium oxide film can be used. Alternatively, a stack of layers of these materials can also be used.

Example 1

FIG. 9 illustrates a configuration example of a semiconductor device of one embodiment of the present invention.

The semiconductor device illustrated in FIG. 9 includes the arithmetic portion 1 performing arithmetic processing and the data storing portion 2 in which data held in the arithmetic portion 1 is stored. The data storing portion 2 includes an inverter 227, an inverter 228, an analog switch 229, an analog switch 230, a transistor 231, a transistor 232, a transistor 233, a transistor 234, a capacitor 235, a capacitor 236, and an inverter 237.

The signal OS_RD and the inverted signal of the signal OS_RD are supplied to the analog switch 229 and the analog switch 230 through the inverter 227 and the inverter 228. Switching of the analog switch 229 and switching of the analog switch 230 are controlled with the signal OS_RD and the inverted signal of the signal OS_RD. Switching of the transistor 231 is controlled with the signal OS_WE that is input to a gate of the transistor 231. When the analog switch 229 and the transistor 231 are on, a potential corresponding to “1” or “0” that is data held in the node (Node_1) is supplied to the capacitor 235 and a gate of the transistor 232 through the analog switch 229 and the transistor 231. When the gate of the transistor 232 corresponds to the node (Node_2), switching of the transistor 232 is controlled with a potential which corresponds to “1” or “0” and is supplied to the node (Node_2).

When the transistor 231 is off, the potential corresponding to “1” or “0” is held in the node (Node_2). It is preferable that the off-state current of the transistor 231 be extremely small. The reason for this is that leakage of electric charge through the transistor 231 from the node (Node_2) can be prevented when the off-state current of the transistor 231 is small.

One of a source and a drain of the transistor 232 is connected to a node to which the power source potential V2 is supplied. The transistor 233 has a function of controlling connection between the other of the source and the drain of the transistor 232 and an input terminal of the inverter 237. The transistor 234 has a function of controlling connection between a node to which the power source potential V1 is supplied and the input terminal of the inverter 237.

The capacitor 236 has a function of holding the potential of a node (Node_3) connecting one of a source and a drain of the transistor 233 and one of a source and a drain of the transistor 234.

In the data storing portion 2, the potential of the node (Node_3) can be reset in such a manner that the transistor 233 is turned off, the transistor 234 is turned on, and the power source potential V1 is supplied to the node (Node_3).

In the data storing portion 2, the potential of the node (Node_3) can reflect data held in the node (Node_2) in such a manner that the transistor 233 is turned on and the transistor 234 is turned off. Specifically, the power source potential V2 is supplied to the node (Node_3) through the transistor 232 when the transistor 232 is on, and the power source potential V1 is held in the node (Node_3) when the transistor 232 is off. In this manner, the potential of the node (Node_3) reflects data held in the node (Node_2) (data input from the arithmetic portion 1).

An output signal of the inverter 237 is supplied to the arithmetic portion 1 through the analog switch 230.

With the above structure, in one embodiment of the present invention, the operation of the data storing portion 2 is controlled in the semiconductor device; thus, data in the arithmetic portion 1 can be stored in the data storing portion 2 and data in the data storing portion 2 can be restored to the arithmetic portion 1.

Example 2

FIG. 10 illustrates a configuration example of a semiconductor device of one embodiment of the present invention.

The semiconductor device illustrated in FIG. 10 includes the arithmetic portion 1 performing arithmetic processing and the data storing portion 2 in which data held in the arithmetic portion 1 is stored. The data storing portion 2 includes a transistor 203, a transistor 204, a capacitor 205, an analog switch 206, a transistor 207, an inverter 208, and an inverter 209.

Switching of the transistor 203 is controlled with the signal OS_WE. Switching of the analog switch 206 is controlled with the signal OS_WE2. Specifically, in FIG. 10, switching of the analog switch 206 is controlled with an output signal of the inverter 208 (the inverted signal of the signal OS_WE2) and an output signal of the inverter 209 (an inverted signal of the output signal of the inverter 208). Switching of the transistor 207 is controlled with the output signal of the inverter 208 (the inverted signal of the signal OS_WE2).

When the analog switch 206 and the transistor 203 are on, a potential corresponding to “1” or “0” that is data held in the node (Node_1) is supplied to the node (Node_2). When the transistor 203 and the transistor 207 are on, the power source potential V2 is supplied to the node (Node_2).

Switching of the transistor 204 is controlled with the potential of the node (Node_2). When the transistor 204 is on, the power source potential V2 is supplied to the arithmetic portion 1 through the transistor 204. When the transistor 203 is off, the capacitor 205 has a function of holding the potential of the node (Node_2).

It is preferable that the off-state current of the transistor 203 be extremely small. The reason for this is that the amount of electric charge leaking from the node (Node_2) can be reduced and thus the length of a period during which data is stored in the data storing portion 2 can be made long when the off-state current of the transistor 203 is small.

Example 3

FIG. 11 shows the measurement results of the time required for writing data to the data storing portion 2 in the semiconductor device illustrated in FIG. 9. FIG. 12 shows the measurement results of the time required for writing data to the data storing portion 2 in the semiconductor device illustrated in FIG. 10. In FIG. 11, a period of time T during which the analog switch 229 and the transistor 231 illustrated in FIG. 9 are on is set to 200 ns (nanoseconds), 400 ns, 600 ns, 800 ns, 1000 ns, 1200 ns, 1400 ns, 1600 ns, 1800 ns, and 2000 ns. Among them, the period of time T at which written data agrees with read data and which is the shortest is set as the time required for writing data. Like in FIG. 11, in FIG. 12, a period of time T during which the analog switch 206 and the transistor 203 illustrated in FIG. 10 are on is set to 200 ns, 400 ns, 600 ns, 800 ns, 1000 ns, 1200 ns, 1400 ns, 1600 ns, 1800 ns, and 2000 ns. Among them, the period of time T at which written data agrees with read data and which is the shortest is set as the time required for writing data.

The time required for writing data in the case where a high-level potential (High) is written to the node (Node_2) and the time required for writing data in the case where a low-level potential (Low) is written to the node (Node_2) were measured. In this measurement, a power source voltage that is a difference between the power source potential V1 and the power source potential V2 was set to 1.6 V, 1.8 V, 2.0 V, 2.2 V, and 2.4 V.

As shown in FIG. 11, the time required for writing data was 1600 ns at a power source voltage of 2.0 V in the semiconductor device illustrated in FIG. 9. On the other hand, as shown in FIG. 12, the time required for writing data was 400 ns at a power source voltage of 2.0 V in the semiconductor device illustrated in FIG. 10. These results indicate that the time required for writing data at a power source voltage of 2.0 V in the semiconductor device illustrated in FIG. 10 is a quarter of that in the semiconductor device illustrated in FIG. 9.

Further, in the semiconductor device illustrated in FIG. 10, the time required for writing data was not changed at room temperature even in the case where data was rewritten 10¹¹ times.

Example 4

FIG. 13 shows the layout of a processor including a memory device in which 255 semiconductor devices illustrated in FIG. 10 are connected. The processor corresponds to one mode of a semiconductor device of one embodiment of the present invention.

In the processor shown in FIG. 13, in the case where an application having a power-source-voltage shut-off time of several tens of microseconds or more is executed, the power-source-voltage shut-off time during which the overhead power consumption and power cut by power-source-voltage shut-off becomes equal in amount, that is, a break even time (BET) was 25.3 microseconds at a power source voltage of 2.0 V and a driving frequency of a CPU of 25 MHz. From the above, a reduction in the power consumption of the processor can be expected when an application having a power-source-voltage shut-off time of several tens of microseconds or more is executed.

Example 5

In this example, referring to FIG. 7, description is given of examples of a structure and a manufacturing method of a semiconductor device including a transistor 902 whose channel is formed in an oxide semiconductor layer and a transistor 901 whose channel is formed in a single crystal silicon wafer. Note that the transistor 902 can be used as the transistor 21 illustrated in FIG. 1A, FIG. 2A, FIG. 3, and FIG. 4A, and the transistor 901 can be used as the transistor 23 illustrated in FIG. 1A, FIG. 2A, FIG. 3, and FIG. 4A, for example.

Note that a semiconductor material such as germanium, silicon germanium, or single crystal silicon carbide as well as silicon may be used for the transistor 901. A transistor including silicon can be formed using a silicon thin film formed by an SOI method or a silicon thin film formed by vapor deposition, for example; in this case, a glass substrate formed by a fusion process or a float process, a quartz substrate, a semiconductor substrate, a ceramic substrate, or the like can be used as a substrate. In the case where a glass substrate is used and the temperature of heat treatment to be performed later is high, it is preferable to use a glass substrate with a strain point of 730° C. or higher.

In the semiconductor device illustrated in FIG. 7, the transistor 901 using a single crystal silicon wafer is formed, and the transistor 902 using an oxide semiconductor is formed above the transistor 901. In other words, the semiconductor device described in this example is a semiconductor device that has a three-dimensional layered structure in which a silicon wafer is used as a substrate and a transistor layer is provided above the silicon wafer. Moreover, the semiconductor device in this example is a hybrid semiconductor device including a transistor in which silicon is used for a channel formation region and a transistor in which an oxide semiconductor is used for a channel formation region.

The transistor 901 formed using a substrate 900 containing a semiconductor material can be either an n-channel transistor (nMOSFET) or a p-channel transistor (pMOSFET). In the example illustrated in FIG. 7, the transistor 901 is electrically isolated from other elements by a shallow trench isolation (STI) 905. The use of the STI 905 can reduce generation of a bird's beak, which may be caused by a LOCOS element isolation method, in an element isolation region and can reduce the size of the element isolation region. On the other hand, in a semiconductor device that is not required to be structurally miniaturized or downsized, the STI 905 is not necessarily formed and an element isolation means such as LOCOS can be used. In the substrate 900 where the transistor 901 is formed, a well 904 to which an impurity imparting conductivity, such as boron, phosphorus, or arsenic, is added is formed.

The transistor 901 in FIG. 7 includes a channel formation region in the substrate 900, impurity regions 906 (also referred to as a source region and a drain region) provided such that the channel formation region is placed therebetween, a gate insulating film 907 over the channel formation region, and a gate electrode layer 908 provided over the gate insulating film 907 to overlap with the channel formation region. The gate electrode layer 908 can have a stacked structure of a gate electrode layer including a first material for increasing processing accuracy and a gate electrode layer including a second material for decreasing the resistance as a wiring. For example, the gate electrode layer can have a stacked structure of crystalline silicon to which an impurity imparting conductivity, such as phosphorus, is added and nickel silicide. Note that the structure is not limited to this, and materials, the number of stacked layers, the shape, or the like can be adjusted as appropriate depending on required specifications.

Note that the transistor 901 illustrated in FIG. 7 may be a fin-type transistor. In a fin-type structure, part of a semiconductor substrate is processed into a plate-shaped protrusion, and a gate electrode layer is provided to cross the protrusion in the longitudinal direction. The gate electrode layer covers an upper surface and side surfaces of the protrusion with a gate insulating film placed between the gate electrode layer and the protrusion. With the second transistor having a fin-type structure, the channel width can be reduced to achieve higher integration of transistors. Moreover, a larger amount of current can flow through the transistor and the control efficiency can be increased, so that the off-state current and threshold voltage of the transistor can be reduced.

Contact plugs 913 and 915 are connected to the impurity regions 906 in the substrate 900. Here, the contact plugs 913 and 915 also function as a source electrode and a drain electrode of the transistor 901. In addition, impurity regions that are different from the impurity regions 906 are provided between the impurity regions 906 and the channel formation region. The impurity regions function as LDD regions or extension regions for controlling the distribution of electric fields in the vicinity of the channel formation region, depending on the concentration of an impurity introduced thereto. Sidewall insulating films 909 are provided at side surfaces of the gate electrode layer 908 with an insulating film placed therebetween. By using this insulating film and the sidewall insulating films 909, the LDD regions or extension regions can be formed.

The transistor 901 is covered with an insulating film 910. The insulating film 910 can function as a protective film and can prevent impurities from entering the channel formation region from the outside. With the insulating film 910 formed by CVD using a material such as silicon nitride, hydrogenation can be performed by heat treatment in the case where single crystal silicon is used for the channel formation region. When an insulating film having tensile stress or compressive stress is used as the insulating film 910, distortion can be provided to the semiconductor material used for the channel formation region. By application of tensile stress to a silicon material used for the channel formation region of an n-channel transistor or application of compressive stress to a silicon material used for the channel formation region of a p-channel transistor, the field-effect mobility of the transistor can be increased.

An insulating film 911 is provided over the insulating film 910, and a surface of the insulating film 911 is planarized by CMP. Consequently, element layers can be stacked with high accuracy above a layer including the transistor 901.

A layer including the transistor 902 in which an oxide semiconductor film is used for a channel is formed above the layer including the transistor 901. The transistor 902 is a top-gate transistor. The transistor 902 includes a source electrode layer 927 and a drain electrode layer 928 in contact with side surfaces and an upper surface of an oxide semiconductor film 926, and includes a gate electrode layer 930 over a gate insulating film 929 placed over the oxide semiconductor film 926, the source electrode layer 927, and the drain electrode layer 928. Insulating films 932 and 933 are formed to cover the transistor 902. Here, a method of fabricating the transistor 902 will be described below.

The oxide semiconductor film 926 is formed over the insulating film 924. The insulating film 924 can be formed using an inorganic insulating film of silicon oxide, silicon nitride, silicon nitride oxide, silicon oxynitride, aluminum oxide, aluminum nitride, aluminum nitride oxide, or the like. In particular, the insulating film 924 is preferably formed using a material with a low dielectric constant (a low-k material) because capacitance due to overlap of electrodes or wirings can be sufficiently reduced. Note that the insulating film 924 may be a porous insulating film containing any of the above materials. Since the porous insulating film has lower dielectric constant than a dense insulating film, parasitic capacitance due to electrodes or wirings can be further reduced. In this example, the insulating film 924 is a stack of a silicon oxide film with a thickness of about 300 nm on a 50-nm-thick aluminum oxide film.

The oxide semiconductor film 926 can be formed by processing an oxide semiconductor film formed over the insulating film 924 into a desired shape. The thickness of the oxide semiconductor film is 2 nm to 200 nm, preferably 3 nm to 50 nm, more preferably 3 nm to 20 nm. The oxide semiconductor film is formed by sputtering using an oxide semiconductor target. Moreover, the oxide semiconductor film can be formed by sputtering under a rare gas (e.g., argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere of a rare gas (e.g., argon) and oxygen.

Note that before the oxide semiconductor film is formed by sputtering, dust on a surface of the insulating film 924 is preferably removed by reverse sputtering in which an argon gas is introduced and plasma is generated. The reverse sputtering refers to a method in which, without application of voltage to a target side, an RF power source is used for application of voltage to a substrate side in an argon atmosphere to generate plasma in the vicinity of the substrate to modify a surface. Note that instead of an argon atmosphere, a nitrogen atmosphere, a helium atmosphere, or the like may be used. Alternatively, an argon atmosphere to which oxygen, nitrous oxide, or the like is added may be used. Further alternatively, an argon atmosphere to which chlorine, carbon tetrafluoride, or the like is added may be used.

In this example, an In—Ga—Zn-based oxide semiconductor thin film that has a thickness of 30 nm and is obtained by sputtering using a target including indium (In), gallium (Ga), and zinc (Zn) is used as the oxide semiconductor film. As the target, it is preferable to use a target having an atomic ratio of In:Ga:Zn=1:1:1, 4:2:3, 3:1:2, 1:1:2, 2:1:3, or 3:1:4. The filling rate of the target including In, Ga, and Zn is higher than or equal to 90% and lower than or equal to 100%, preferably higher than or equal to 95% and lower than 100%. With the use of the target with high filling rate, a dense oxide semiconductor film is formed.

In the case where an In—Zn-based material is used for the oxide semiconductor film, a target to be used has a composition ratio of In:Zn=50:1 to 1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio), preferably In:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2 in a molar ratio), more preferably In:Zn=15:1 to 1.5:1 in an atomic ratio (In₂O₃:ZnO=15:2 to 3:4 in a molar ratio). For example, in a target used for formation of an In—Zn-based oxide semiconductor with an atomic ratio of In:Zn:O=X:Y:Z, the relation of Z>1.5X+Y is satisfied. The mobility can be improved by keeping the ratio of Zn within the above range.

In the case where an In—Sn—Zn-based oxide semiconductor film is formed as the oxide semiconductor film by sputtering, it is preferable to use an In—Sn—Zn—O target having an atomic ratio of In:Sn:Zn=1:1:1, 2:1:3, 1:2:2, or 20:45:35.

In this example, the oxide semiconductor film is formed in such a manner that the substrate is held in a process chamber kept in a reduced pressure state, a sputtering gas from which hydrogen and moisture are removed is introduced while moisture remaining in the process chamber is removed, and the above-described target is used. The substrate temperature during the deposition may be 100° C. to 600° C., preferably 200° C. to 400° C. By forming the oxide semiconductor film while the substrate is heated, the concentration of impurities included in the formed oxide semiconductor film can be reduced. In addition, damage by sputtering can be reduced. In order to remove remaining moisture in the process chamber, an entrapment vacuum pump is preferably used. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. The evacuation unit may be a turbo pump provided with a cold trap. In the process chamber which is evacuated with the cryopump, for example, a hydrogen atom, a compound containing a hydrogen atom, such as water (H₂O), and the like are removed, whereby the impurity concentration in the oxide semiconductor film formed in the process chamber can be reduced.

As one example of the deposition conditions, the distance between the substrate and the target is 100 mm, the pressure is 0.6 Pa, the direct-current (DC) power is 0.5 kW, and the atmosphere is an oxygen atmosphere (the proportion of the oxygen flow rate is 100%). Note that a pulsed direct-current (DC) power source is preferably used because dust generated in deposition can be reduced and the film thickness can be made uniform.

By setting the leakage rate of the process chamber of the sputtering apparatus to 1×10⁻¹⁰ Pa·m³/s or less, impurities, such as an alkali metal or a hydride, which enter the oxide semiconductor film that is being formed by sputtering can be reduced. Further, with the use of an entrapment vacuum pump as an exhaustion system, counter flow of impurities such as an alkali metal, a hydrogen atom, a hydrogen molecule, water, or a hydride from the exhaustion system can be reduced.

When the purity of the target is set to 99.99% or higher, an alkali metal, a hydrogen atom, a hydrogen molecule, water, a hydroxyl group, a hydride, or the like entering the oxide semiconductor film can be reduced. In addition, when the above target is used, the concentration of an alkali metal such as lithium, sodium, or potassium can be reduced in the oxide semiconductor film.

In order that the oxide semiconductor film contains hydrogen, a hydroxyl group, and moisture as little as possible, it is preferable that impurities such as moisture or hydrogen that are adsorbed on the substrate 900 be desorbed and exhausted by preheating of the substrate 900 over which the insulating film 924 is formed in a preheating chamber of a sputtering apparatus, as pretreatment for deposition. The temperature for the preheating is 100° C. to 400° C., preferably 150° C. to 300° C. As an evacuation unit provided in the preheating chamber, a cryopump is preferably used. Note that this preheating treatment is not necessarily performed.

Note that etching for forming the oxide semiconductor film 926 may be dry etching, wet etching, or both dry etching and wet etching. As an etching gas used for dry etching, it is preferable to use a gas containing chlorine (a chlorine-based gas such as chlorine (Cl₂), boron trichloride (BCl₃), silicon tetrachloride (SiCl₄), or carbon tetrachloride (CCl₄)). Alternatively, it is possible to use a gas containing fluorine (a fluorine-based gas such as carbon tetrafluoride (CF₄), sulfur hexafluoride (SF₆), nitrogen trifluoride (NF₃), or trifluoromethane (CHF₃)), hydrogen bromide (HBr), oxygen (O₂), any of these gases to which a rare gas such as helium (He) or argon (Ar) is added, or the like.

As the dry etching method, a parallel plate reactive ion etching (RIE) method or an inductively coupled plasma (ICP) etching method can be used. In order to etch the film into a desired shape, the etching conditions (e.g., the electric power applied to a coiled electrode, the electric power applied to an electrode on the substrate side, and the electrode temperature on the substrate side) are adjusted as appropriate.

A resist mask used for forming the oxide semiconductor film 926 may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, fabrication cost can be reduced.

Note that it is preferable that reverse sputtering be performed before the formation of a conductive film in a subsequent step so that a resist residue and the like that attach onto surfaces of the oxide semiconductor film 926 and the insulating film 924 are removed.

Note that the oxide semiconductor film formed by sputtering or the like sometimes contains a large amount of moisture or hydrogen (including a hydroxyl group) as impurities. Moisture or hydrogen easily forms donor levels and thus serves as impurities in the oxide semiconductor. Therefore, in this example of the present invention, in order to reduce impurities such as moisture and hydrogen in the oxide semiconductor film (dehydrate or dehydrogenate the oxide semiconductor film), the oxide semiconductor film 926 is subjected to heat treatment in a reduced-pressure atmosphere, an inert gas atmosphere of nitrogen, a rare gas, or the like, an oxygen gas atmosphere, or an ultra-dry air atmosphere (with a moisture content of 20 ppm (−55° C. by conversion into a dew point) or less, preferably 1 ppm or less, more preferably 10 ppb or less, in the case where the measurement is performed by a dew point meter of a cavity ring down laser spectroscopy (CRDS) system).

By performing heat treatment on the oxide semiconductor film 926, moisture or hydrogen in the oxide semiconductor film 926 can be eliminated. Specifically, heat treatment may be performed at a temperature higher than or equal to 250° C. and lower than or equal to 750° C., preferably higher than or equal to 400° C. and lower than the strain point of the substrate. For example, heat treatment may be performed at 500° C. for approximately 3 minutes to 6 minutes. When RTA is used for the heat treatment, dehydration or dehydrogenation can be performed in a short time; thus, treatment can be performed even at a temperature higher than the strain point of a glass substrate.

In this example, an electric furnace that is one of heat treatment apparatuses is used.

Note that the heat treatment apparatus is not limited to an electric furnace, and may have a device for heating an object by heat conduction or heat radiation from a heating element such as a resistance heating element. For example, a rapid thermal annealing (RTA) apparatus such as a gas rapid thermal annealing (GRTA) apparatus or a lamp rapid thermal annealing (LRTA) apparatus can be used. An LRTA apparatus is an apparatus for heating an object by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high pressure mercury lamp. A GRTA apparatus is an apparatus for heat treatment using a high-temperature gas. As the gas, an inert gas which does not react with an object by heat treatment, like nitrogen or a rare gas such as argon, is used.

In the heat treatment, it is preferable that moisture, hydrogen, and the like be not contained in nitrogen or a rare gas such as helium, neon, or argon. Alternatively, the purity of nitrogen or a rare gas such as helium, neon, or argon which is introduced into the heat treatment apparatus is preferably 6N (99.9999%) or higher, more preferably 7N (99.99999%) or higher (i.e., the impurity concentration is preferably 1 ppm or lower, more preferably 0.1 ppm or lower).

Through the above-described steps, the concentration of hydrogen in the oxide semiconductor film 926 can be reduced and the oxide semiconductor film 926 can be highly purified. Thus, the oxide semiconductor film can be stable. In addition, by using the oxide semiconductor film in which the hydrogen concentration is reduced and the purity is improved, it is possible to fabricate a transistor with high withstand voltage and extremely small off-state current. The above heat treatment can be performed at any time after the oxide semiconductor film is formed.

Next, the source electrode layer 927 and the drain electrode layer 928 are formed by a photolithography process. Specifically, the source electrode layer 927 and the drain electrode layer 928 can be formed in such a manner that a conductive film is formed over the insulating film 924 by sputtering or vacuum evaporation and then processed (patterned) into a predetermined shape.

In this example, a 100-nm-thick tungsten film is used for the source electrode layer 927 and the drain electrode layer 928.

Note that the materials and etching conditions are adjusted as appropriate so that the oxide semiconductor film 926 is not removed as much as possible in etching of the conductive film. Depending on the etching conditions, an exposed portion of the oxide semiconductor film 926 is partially etched and thus a groove (a depressed portion) is formed in some cases.

In this example, a tungsten film is used as the conductive film to be the source electrode layer 927 and the drain electrode layer 928. Thus, wet etching can be selectively performed on the conductive film using a solution containing ammonia and hydrogen peroxide water (an ammonia hydrogen peroxide mixture). As the ammonia hydrogen peroxide mixture, a solution in which 31 wt % hydrogen peroxide water, 28 wt % ammonia water, and water are mixed at a volume ratio of 5:2:2 is specifically used. Alternatively, dry etching may be performed on the conductive film with the use of a gas containing carbon tetrafluoride (CF₄), chlorine (Cl₂), or oxygen.

In order to reduce the number of photomasks and steps in a photolithography process, etching may be performed with the use of a resist mask formed using a multi-tone mask which is a light-exposure mask through which light is transmitted so as to have a plurality of intensities. A resist mask formed using a multi-tone mask has a plurality of thicknesses and can be changed in shape by ashing; thus, the resist mask can be used in a plurality of etching steps for processing films into different patterns. That is, a resist mask corresponding to at least two kinds of different patterns can be formed by one multi-tone mask. As a result, the number of light-exposure masks can be reduced and the number of corresponding photolithography processes can also be reduced, whereby the fabrication process can be simplified.

Further, an oxide conductive film functioning as a source region and a drain region may be provided between the oxide semiconductor film 926 and the source and drain electrode layers 927 and 928. The material of the oxide conductive film preferably contains zinc oxide as a component and preferably does not contain indium oxide. For such an oxide conductive film, zinc oxide, zinc aluminum oxide, zinc aluminum oxynitride, zinc gallium oxide, or the like can be used.

For example, in the case where the oxide conductive film is formed, patterning for forming the oxide conductive film and patterning for forming the source electrode layer 927 and the drain electrode layer 928 may be performed concurrently.

By providing the oxide conductive film functioning as the source and drain regions, the resistance between the oxide semiconductor film 926 and the source and drain electrode layers 927 and 928 can be lowered, so that the transistor can operate at high speed. In addition, by providing the oxide conductive film functioning as the source and drain regions, the withstand voltage of the transistor can be increased.

Next, plasma treatment may be performed using a gas such as N₂O, N₂, or Ar. By this plasma treatment, water or the like attached onto an exposed surface of the oxide semiconductor film is removed. Plasma treatment may be performed using a mixture gas of oxygen and argon.

After the plasma treatment, the gate insulating film 929 is formed to cover the source and drain electrode layers 927 and 928 and the oxide semiconductor film 926. Then, over the gate insulating film 929, the gate electrode layer 930 is formed to overlap with the oxide semiconductor film 926.

In this example, a 20-nm-thick silicon oxynitride film formed by sputtering is used as the gate insulating film 929. The substrate temperature during the film formation may range from room temperature to 400° C. and is 300° C. in this example.

After the gate insulating film 929 is formed, heat treatment may be performed. The heat treatment is performed in a nitrogen atmosphere, ultra-dry air, or a rare gas (e.g., argon or helium) atmosphere preferably at 200° C. to 400° C., and for example at 250° C. to 350° C. It is preferable that the water content in the gas be 20 ppm or less, preferably 1 ppm or less, more preferably 10 ppb or less. In this example, for example, heat treatment is performed in a nitrogen atmosphere at 250° C. for 1 hour. Alternatively, RTA treatment for a short time at a high temperature may be performed before the formation of the source and drain electrode layers 927 and 928 in a manner similar to that of the heat treatment performed on the oxide semiconductor film for reduction of moisture or hydrogen. Even when oxygen vacancies are generated in the oxide semiconductor film 926 by the previous heat treatment performed on the oxide semiconductor film 926, oxygen is supplied to the oxide semiconductor film 926 from the gate insulating film 929 by performing heat treatment after the gate insulating film 929 containing oxygen is provided. By the supply of oxygen to the oxide semiconductor film 926, oxygen vacancies that serve as donors can be reduced in the oxide semiconductor film 926 and the stoichiometric composition can be satisfied. As a result, the oxide semiconductor film 926 can be made to be substantially i-type and variations in electrical characteristics of the transistor due to oxygen vacancies can be reduced; thus, electrical characteristics can be improved. There is no particular limitation on the timing of this heat treatment as long as it is after the formation of the gate insulating film 929. When this heat treatment doubles as another step, the oxide semiconductor film 926 can be made to be substantially i-type without increase in the number of steps.

Moreover, oxygen vacancies that serve as donors in the oxide semiconductor film 926 may be reduced by subjecting the oxide semiconductor film 926 to heat treatment in an oxygen atmosphere so that oxygen is added to the oxide semiconductor. The heat treatment is performed at a temperature, for example, higher than or equal to 100° C. and lower than 350° C., preferably higher than or equal to 150° C. and lower than 250° C. It is preferable that an oxygen gas used for the heat treatment in an oxygen atmosphere do not include water, hydrogen, or the like. Alternatively, the purity of the oxygen gas which is introduced into the heat treatment apparatus is preferably 6N (99.9999%) or higher, more preferably 7N (99.99999%) or higher (i.e., the impurity concentration in the oxygen gas is preferably 1 ppm or lower, more preferably 0.1 ppm or lower).

Alternatively, oxygen may be added to the oxide semiconductor film 926 by ion implantation, ion doping, or the like to reduce oxygen vacancies serving as donors. For example, oxygen that is made into a plasma state with a microwave at 2.45 GHz may be added to the oxide semiconductor film 926.

The gate electrode layer 930 can be formed in such a manner that a conductive film is formed over the gate insulating film 929 and then is patterned.

The thickness of the gate electrode layer 930 is 10 nm to 400 nm, preferably 100 nm to 300 nm. In this example, the gate electrode layer 930 is formed in the following manner: a 135-nm-thick tungsten film is stacked over a 30-nm-thick tantalum nitride film by sputtering to form a conductive film for the gate electrode, and then, the conductive film is processed (patterned) into a desired shape by etching. Note that a resist mask may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, fabrication cost can be reduced.

Through the above steps, the transistor 902 is formed.

Note that the transistor 902 is described as a single-gate transistor; if necessary, it is possible to fabricate a multi-gate transistor that includes a plurality of gate electrodes electrically connected to each other and thus includes a plurality of channel formation regions.

In the fabrication method described above, the source electrode layer 927 and the drain electrode layer 928 are formed after the oxide semiconductor film 926. Accordingly, as illustrated in FIG. 7, in the transistor 902 obtained by the above-described method, the source electrode layer 927 and the drain electrode layer 928 are formed over the oxide semiconductor film 926. Alternatively, in the transistor 902, the source and drain electrode layers 927 and 928 may be formed below the oxide semiconductor film 926, that is, between the oxide semiconductor film 926 and the insulating film 924.

Note that an insulating film in contact with the oxide semiconductor film 926 may be formed using an insulating material containing an element that belongs to Group 13 and oxygen. Many of oxide semiconductor materials contain a Group 13 element, and an insulating material containing a Group 13 element is compatible with an oxide semiconductor. Thus, when an insulating material containing a Group 13 element is used for the insulating film in contact with the oxide semiconductor film, the state of the interface between the oxide semiconductor film and the insulating film can be kept favorable.

An insulating material containing a Group 13 element is an insulating material containing one or more elements that belong to Group 13. As the insulating material containing a Group 13 element, gallium oxide, aluminum oxide, aluminum gallium oxide, gallium aluminum oxide, and the like are given. Here, the amount of aluminum is larger than that of gallium in atomic percent in aluminum gallium oxide, whereas the amount of gallium is larger than or equal to that of aluminum in atomic percent in gallium aluminum oxide.

For example, in the case of forming an insulating film in contact with an oxide semiconductor film containing gallium, a material containing gallium oxide may be used for the insulating film, so that favorable characteristics can be kept at the interface between the oxide semiconductor film and the insulating film. For example, the oxide semiconductor film and an insulating film containing gallium oxide are provided in contact with each other, so that pileup of hydrogen at the interface between the oxide semiconductor film and the insulating film can be reduced. Note that a similar effect can be obtained in the case where an element in the same group as a constituent element of the oxide semiconductor is used in the insulating film. For example, it is effective to form an insulating film with the use of a material containing aluminum oxide. Note that water is less likely to permeate aluminum oxide, and it is therefore preferable to use a material containing aluminum oxide in terms of preventing entry of water to the oxide semiconductor film.

The insulating film which is in contact with the oxide semiconductor film 926 preferably contains oxygen in a proportion higher than the stoichiometric composition by heat treatment in an oxygen atmosphere, oxygen doping, or the like. “Oxygen doping” refers to addition of oxygen into a bulk. Note that the term “bulk” is used in order to clarify that oxygen is added not only to a surface of a thin film but also to the inside of the thin film. In addition, “oxygen doping” includes “oxygen plasma doping” in which oxygen which is made to be plasma is added to a bulk. The oxygen doping may be performed by ion implantation or ion doping.

By oxygen doping, an insulating film that includes a region where the proportion of oxygen is higher than that in the stoichiometric composition can be formed. When the insulating film including such a region is in contact with the oxide semiconductor film, excess oxygen in the insulating film is supplied to the oxide semiconductor film, and oxygen vacancies in the oxide semiconductor film or at the interface between the oxide semiconductor film and the insulating film are reduced. Thus, the oxide semiconductor film can be made to be i-type or substantially i-type.

Note that the insulating film including a region where the proportion of oxygen is higher than that in the stoichiometric composition may be used as either the insulating film placed above the oxide semiconductor film 926 or the insulating film placed below the oxide semiconductor film 926 of the insulating films in contact with the oxide semiconductor film 926; however, it is preferable to use such an insulating film as both of the insulating films in contact with the oxide semiconductor film 926. The above-described effect can be enhanced with a structure in which the insulating films each including a region where the proportion of oxygen is higher than that in the stoichiometric composition are used as the insulating films in contact with the oxide semiconductor film 926 and positioned above and below the oxide semiconductor film 926 so that the oxide semiconductor film 926 is sandwiched between the insulating films.

The constituent elements may be the same or different between the insulating films placed above and below the oxide semiconductor film 926. The insulating film in contact with the oxide semiconductor film 926 may be a stack of insulating films each including a region where the proportion of oxygen is higher than that in the stoichiometric composition.

Note that in this example, the transistor 902 has a top-gate structure. The transistor 902 includes a backgate electrode layer 923. With the backgate electrode layer, the transistor 902 can serve as a normally-off transistor. For example, when the potential of the backgate electrode layer 923 is set at GND or a fixed potential, the threshold voltage of the transistor 902 can shift further in a positive direction, which leads to the formation of a normally-off transistor.

In order to electrically connect the transistor 901 and the transistor 902 to form an electric circuit, one or more wiring layers for connecting these elements are stacked between layers and on the upper layer.

In FIG. 7, for example, one of the source and the drain of the transistor 901 is electrically connected to a wiring layer 914 through the contact plug 913. The other of the source and the drain of the transistor 901 is electrically connected to a wiring layer 916 through the contact plug 915. The gate of the transistor 901 is electrically connected to the drain electrode layer 928 of the transistor 902 through a contact plug 917, a wiring layer 918, a contact plug 921, a wiring layer 922, and a contact plug 925.

The wiring layers 914, 916, 918, and 922, and the backgate electrode layer 923 are embedded in insulating films. These wiring layers and the like are preferably formed using a low-resistance conductive material such as copper or aluminum. Alternatively, the wiring layers can be formed using graphene formed by CVD as a conductive material. Graphene is a one-atom thick sheet of sp²-bonded carbon molecules or a stack of 2 to 100 sheets of the carbon molecules. Examples of a method of manufacturing such graphene are thermal CVD by which graphene is formed on a metal catalyst; and plasma CVD by which graphene is formed from methane, without using a catalyst, by plasma generated locally with ultraviolet light irradiation.

By using such a low-resistance conductive material, RC delay of signals transmitted through the wiring layers can be reduced. When copper is used for the wiring layers, a barrier film is formed in order to prevent copper from diffusing into the channel formation region. The barrier film can be a tantalum nitride film, a stack of a tantalum nitride film and a tantalum film, a titanium nitride film, or a stack of a titanium nitride film and a titanium film, for example, but is not limited to a film containing such materials as long as the film has a function of preventing diffusion of a wiring material and has adhesion to the wiring material, a base film, or the like. The barrier film may be formed as a layer that is independently formed, or may be formed in such a manner that a material of the barrier film is included in a wiring material and precipitated by heat treatment on the inner wall of an opening provided in an insulating film.

The insulating films 911, 912, 919, 920, and 933 can be formed using an insulator such as silicon oxide, silicon oxynitride, silicon nitride oxide, borophosphosilicate glass (BPSG), phosphosilicate glass (PSG), silicon oxide to which carbon is added (SiOC), silicon oxide to which fluorine is added (SiOF), tetraethylorthosilicate (TEOS) which is silicon oxide prepared from Si(OC₂H₅)₄, hydrogen silsesquioxane (HSQ), methyl silsesquioxane (MSQ), organosilicate glass (OSG), or an organic polymer-based material. In particular, in the case of advancing miniaturization of a semiconductor device, parasitic capacitance between wirings is significant and signal delay is increased; therefore, the relative permittivity of silicon oxide (k=4.0 to 4.5) is too high, and a material with k=3.0 or less is preferably used. In addition, since CMP treatment is performed after the wirings are embedded in the insulating films, the insulating films need to have high mechanical strength. The insulating films can be made porous to have a lower dielectric constant as long as their mechanical strength can be secured. The insulating films are formed by sputtering, CVD, a coating method including spin coating (also referred to as spin on glass (SOG)), or the like.

An insulating film functioning as an etching stopper for planarization treatment by CMP or the like that is performed after the wiring material is embedded in the insulating films 911, 912, 919, 920, and 933 may be additionally provided.

Each of the contact plugs 913, 915, 917, 921, and 925 is formed in such a manner that an opening (a via hole) with a high aspect ratio is formed in the insulating film and is filled with a conductive material such as tungsten. The opening is formed preferably by highly anisotropic dry etching and particularly preferably by reactive ion etching (RIE). The inner wall of the opening is covered with a barrier film (diffusion prevention film) formed of a titanium film, a titanium nitride film, a stack of such films, or the like, and a material such as tungsten or polysilicon doped with phosphorus or the like fills the opening. For example, tungsten is embedded in the via hole by blanket CVD, and an upper surface of the contact plug is planarized by CMP.

Example 6

The semiconductor device of one embodiment of the present invention can be used for electronic devices in a wide variety of fields, such as digital signal processing, software-defined radio systems, avionic systems (electronic devices used in aircraft, such as communication systems, navigation systems, autopilot systems, and flight management systems), ASIC prototyping, medical image processing, voice recognition, encryption, bioinformatics, emulators for mechanical systems, and radio telescopes in radio astronomy.

Examples of consumer products among such electronic devices are display devices, personal computers, and image reproducing devices provided with recording media (devices that reproduce the content of recording media such as digital versatile discs (DVDs) and have displays for displaying the reproduced images). Other examples of electronic devices that can include the semiconductor device of one embodiment of the present invention are mobile phones, game consoles including portable game consoles, portable information terminals, e-book readers, cameras such as video cameras and digital still cameras, goggle-type displays (head mounted displays), navigation systems, audio reproducing devices (e.g., car audio systems and digital audio players), copiers, facsimiles, printers, and multifunction printers. FIGS. 8A to 8F illustrate specific examples of these electronic devices.

FIG. 8A illustrates a portable game console. The portable game console illustrated in FIG. 8A includes a housing 5001, a housing 5002, a display portion 5003, a display portion 5004, a microphone 5005, a speaker 5006, an operation key 5007, a stylus 5008, and the like. Note that although the portable game console illustrated in FIG. 8A has the two display portions 5003 and 5004, the number of display portions included in the portable game console is not limited to two.

FIG. 8B illustrates a portable information terminal. The portable information terminal illustrated in FIG. 8B includes a first housing 5601, a second housing 5602, a first display portion 5603, a second display portion 5604, a joint 5605, an operation key 5606, and the like. The first display portion 5603 is provided in the first housing 5601, and the second display portion 5604 is provided in the second housing 5602. The first housing 5601 and the second housing 5602 are connected to each other with the joint 5605, and the angle between the first housing 5601 and the second housing 5602 can be changed with the joint 5605. Images on the first display portion 5603 may be switched depending on the angle at the joint 5605 between the first housing 5601 and the second housing 5602. A display device with a position input function may be used as at least one of the first display portion 5603 and the second display portion 5604. Note that the position input function can be added by providing a touch panel in a display device. Alternatively, the position input function can be added by providing a photoelectric conversion element called a photosensor in a pixel area of a display device.

FIG. 8C illustrates a laptop personal computer. The laptop personal computer illustrated in FIG. 8C includes a housing 5401, a display portion 5402, a keyboard 5403, a pointing device 5404, and the like.

FIG. 8D illustrates an electric refrigerator-freezer. The electric refrigerator-freezer illustrated in FIG. 8D includes a housing 5301, a refrigerator door 5302, a freezer door 5303, and the like.

FIG. 8E illustrates a video camera. The video camera illustrated in FIG. 8E includes a first housing 5801, a second housing 5802, a display portion 5803, operation keys 5804, a lens 5805, a joint 5806, and the like. The operation keys 5804 and the lens 5805 are provided in the first housing 5801, and the display portion 5803 is provided in the second housing 5802. The first housing 5801 and the second housing 5802 are connected to each other with the joint 5806, and the angle between the first housing 5801 and the second housing 5802 can be changed with the joint 5806. Images displayed on the display portion 5803 may be switched depending on the angle at the joint 5806 between the first housing 5801 and the second housing 5802.

FIG. 8F illustrates a passenger car. The passenger car illustrated in FIG. 8F includes a car body 5101, wheels 5102, a dashboard 5103, lights 5104, and the like.

This application is based on Japanese Patent Application serial no. 2012-152318 filed with Japan Patent Office on Jul. 6, 2012, and Japanese Patent Application serial no. 2013-004143 filed with Japan Patent Office on Jan. 14, 2013, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A semiconductor device comprising: a memory cell comprising: a first node; a second node; a first transistor comprising a channel formation region in an oxide semiconductor layer; a second transistor; and a circuit for supplying a potential corresponding to 1 or 0 to the second node, wherein one of a source and a drain of the first transistor is electrically connected to the first node, wherein the other of the source and the drain of the first transistor is electrically connected to the second node, and wherein a gate of the second transistor is electrically connected to the other of the source and the drain of the first transistor.
 2. The semiconductor device according to claim 1, wherein the first node is included in an arithmetic portion, and wherein the second node is included in a data storing portion.
 3. The semiconductor device according to claim 1, wherein the circuit comprises: a first switch; a second switch; an inverter; and a wiring supplying the potential corresponding to 1 or 0, wherein the first switch is electrically connected to the first node and the one of the source and the drain of the first transistor, wherein the second switch is electrically connected to the wiring, the first switch and the one of the source and the drain of the first transistor, and wherein the inverter is electrically connected to the first switch and the second switch.
 4. The semiconductor device according to claim 3, wherein the second switch is a transistor.
 5. The semiconductor device according to claim 1, wherein the circuit comprises; a third transistor comprising a channel formation region in an oxide semiconductor layer; and a wiring supplying the potential corresponding to 1 or 0, wherein one of a source and a drain of the third transistor is electrically connected to the wiring, and wherein the other of the source and the drain of the third transistor is electrically connected to the second node.
 6. The semiconductor device according to claim 1, wherein the first transistor is provided over the second transistor with an insulating layer therebetween, and wherein the second transistor comprises a channel formation region in a semiconductor layer comprising silicon.
 7. A semiconductor device comprising: a memory cell comprising: a first node; a first transistor electrically connected to the first node, wherein the first transistor comprises a channel formation region in an oxide semiconductor layer; a second node electrically connected to the first transistor; a second transistor electrically connected to the second node; and a circuit for supplying a potential corresponding to 1 or 0 to the second node, wherein the first node is capable of holding the potential corresponding to 1 or 0 as data in a period during which a power source voltage is supplied to the semiconductor device, wherein the second node is capable of holding the data regardless of whether the power source voltage is supplied, wherein the first transistor is configured to control whether to electrically connect the first node and the second node, and wherein an on/off state of the second transistor depends on a potential of the second node.
 8. The semiconductor device according to claim 7, wherein the circuit comprises: a switch; and a wiring supplying the potential corresponding to 1 or 0, wherein the switch is configured to electrically connect one of a source and a drain of the first transistor either to the first node or to the wiring, and wherein the potential corresponding to 1 or 0 is supplied to the second node when the one of the source and the drain of the first transistor is electrically connected to the wiring and the first transistor is on.
 9. The semiconductor device according to claim 7, wherein the circuit comprises: a third transistor comprising a channel formation region in an oxide semiconductor layer; and a wiring supplying the potential corresponding to 1 or 0, wherein the third transistor is configured to control whether to electrically connect the second node and the wiring, and wherein the potential corresponding to 1 or 0 is supplied to the second node when the third transistor is on.
 10. The semiconductor device according to claim 7, wherein the first transistor is provided over the second transistor with an insulating layer therebetween, and wherein the second transistor comprises a channel formation region in a semiconductor layer comprising silicon.
 11. A driving method of a semiconductor device comprising: the semiconductor device comprising a memory cell comprising: a first node; a first transistor electrically connected to the first node, wherein the first transistor comprises a channel formation region in an oxide semiconductor layer; a second node electrically connected to the first transistor; and a second transistor electrically connected the second node, and the driving method comprising: a first step of supplying a potential corresponding to 1 or 0 to the second node; a second step of turning on the first transistor after the first step to store data held in the first node in the second node; a third step of stopping supply of a power source voltage to the semiconductor device after the second step; a fourth step of restarting the supply of the power source voltage to the semiconductor device after the third step; and a fifth step of restoring the data stored in the second node in the first node depending on an on/off state of the second transistor after the fourth step.
 12. The driving method of the semiconductor device according to claim 11, wherein a length of a period during which the first step is performed is longer than a length of a period during which the second step is performed.
 13. The driving method of the semiconductor device according to claim 11, wherein the semiconductor device further comprises: a switch; and a wiring supplying the potential corresponding to 1 or 0, and wherein the first step is performed by electrically connecting the first transistor and the wiring by the switch.
 14. The driving method of the semiconductor device according to claim 11, wherein the semiconductor device further comprises: a third transistor comprising a channel formation region in an oxide semiconductor layer; and a wiring supplying the potential corresponding to 1 or 0, and wherein the first step is performed by electrically connecting the second node and the wiring by the third transistor.
 15. The driving method of the semiconductor device according to claim 11, wherein the first transistor is provided over the second transistor with an insulating layer therebetween, and wherein the second transistor comprises a channel formation region in a semiconductor layer comprising silicon.
 16. A semiconductor device comprising: a memory cell comprising: a first node capable of holding data in a period during which a power source voltage is supplied; a second node capable of holding the data regardless of whether the power source voltage is supplied; a first transistor of which one of a source and a drain is electrically connected to the second node, comprising a channel formation region in an oxide semiconductor layer; a second transistor of which a gate is electrically connected to the second node; a capacitor of which one terminal is electrically connected to the second node; and a circuit electrically connected to the first node, comprising: a wiring supplying a potential corresponding to 1 or 0; and a switch configured to switch between a first state in which the other of the source and the drain of the first transistor is electrically connected to the first node and a second state in which the other of the source and the drain of the first transistor is electrically connected to the wiring.
 17. The semiconductor device according to claim 16, wherein when the second state is selected by the switch and the first transistor is in an on state, the potential corresponding to 1 or 0 is supplied to the second node regardless of the data.
 18. The semiconductor device according to claim 16, wherein the first transistor is provided over the second transistor with an insulating layer therebetween, and wherein the second transistor comprises a channel formation region in a semiconductor layer comprising silicon. 